1. Field of the Invention
The present invention relates to an exposure apparatus for manufacturing, for example, semiconductor integrated circuits and liquid crystal device substrates, and a positioning method for positioning the circuits and substrates.
2. Description of the Prior Arts
Step-and-repeat reduction projection exposure apparatuses (hereafter referred to as "steppers") play a central role in lithography processes in the manufacture of semiconductor integrated circuits. As alignment mark detection means used in the steppers to align the projected image of a circuit pattern formed on a reticle (that may hereafter be represented by a "mask" that is a superordinate concept of the reticle) with a circuit pattern (hereafter referred to as a chip) already formed on a photosensitive substrate (hereafter referred to as a "wafer"), there exist a laser step alignment system (LSA system) which receives a diffracted and scattered light from an alignment mark on a wafer by relatively moving a slit-like laser beam and the alignment mark, and a field image alignment system (FIA system) which takes an image of an alignment mark on a wafer using a TV camera.
An LSA system of a TTL (through-the-lens) type projects a spot light with a cross section like a band on a wafer via a projection optical system. The spot light scans the wafe r placed on a stage, and a scattering light from an alignment mark on the wafer, which is g enerated when the alignment mark crosses the spot light, are detected to locate the mark.
An FIA system of an OFF-AXIS type projects light with multiple wave lengths against the alignment mark on the wafer, detects reflected light using an image sensor, and image-processes the output from the image sensors to detect the position of the mark.
Since conventional alignment systems detect the position of an alignment mark on a photosensitive substrate by projecting light onto the mark, detection errors often occur due to interference by a photosensitive layer on the mark. FIA system is subject to errors in detecting marks with a small height, while LSA system suffer from errors in detecting asymmetric marks. A first problem is t hat such detection errors reduce the accuracy of registration. Registration is a technique for maintaining a projected image of pattern on a mask and a mark on the photosensitive substrate in a specified relationship when the pattern is projected on a substrate by carrying out alignment using the mark as an index.
A second problem is that a registration error (positioning error) occurring during exposure cannot be detected until development is finished. A third problem is that the method for detecting a registration error after development is time-consuming.